The present invention relates to an ultrasonic wire bonder used in manufacturing semiconductor devices.
Ultrasonic wire bonders have been widely used particularly in the field of semiconductor devices. For example, a gold wire is bonded for connection between an inner lead of a lead frame and a bonding pad on a semiconductor chip die-bonded on the lead frame. Wire is generally bonded by using both ultrasonic bonding and heat-pressurized bonding.
In ultrasonic bonding, ultrasonic energy of approx. 60 kHz generated from an ultrasonic oscillator is propagated via an ultrasonic horn and a capillary to a gold wire so that an ultrasonic wave having an amplitude of 1 to 3 microns is applied to the gold wire. In heat-pressurized bonding, a heater mounted within a workpiece support base is raised to a temperature of about 200 to 250 degrees centigrade while a load about several tens to one hundred grams is applied through an ultrasonic horn.
Ultrasonic bonding, however, is associated with the following problems. An ultrasonic wave to be applied to a gold wire has directivity in that it is applied only in a direction parallel to an Y axis which is the longitudinal direction of an ultrasonic horn. Therefore, the bonding quality changes depending upon the bonding direction along which a gold wire is extended and upon the longitudinal direction of an inner lead of a lead frame. In the case of bonding a wire to a pad on a semiconductor chip, ultrasonic energy is propagated most efficiently if the bonding direction is parallel to the Y axis which is the direction of ultrasonic wave vibration. In the case of bonding a wire to an inner lead, ultrasonic energy is propagated most efficiently if the longitudinal direction of the inner lead is parallel to the Y axis.
Accordingly, ultrasonic energy is propagated sufficiently to a gold wire and an inner lead if both the bonding direction and the inner lead longitudinal direction are parallel to the Y axis, to thereby ensure a sufficiently high bonding force. On the other hand, if both the bonding direction and the inner lead longitudinal direction are orthogonal to the Y axis, ultrasonic energy is not propagated sufficiently to a gold wire and an inner lead so that a high bonding force cannot be obtained. As above, a bonding force fluctuates depending upon the bonding direction for bonding to the pad, and upon the inner lead longitudinal direction for bonding to the inner lead.
The bonding direction along which a gold wire is extended, is generally coincident with the inner lead longitudinal direction. However, both the directions become non-coincident with each other if an inner lead is mounted obliquely relative to the lead frame side. In such a case, the inner lead longitudinal direction is not coincident with the direction of ultrasonic vibration so that a sufficient bonding force cannot be obtained.
If the direction of ultrasonic vibration is not coincident with the bonding direction or the inner lead longitudinal direction, a bonding force becomes insufficient resulting in a problem of deteriorating reliability of semiconductor devices.